According to the prior art for RF MEMS micro-switches, RF switching is obtained by varying the capacitance of a capacitor whose plates are formed, on the one hand, by a membrane and, on the other, by a facing control electrode, a dielectric being provided between the two capacitor plates generally on the electrode. The capacitance then varies from a value Con to a value Coff. The dielectric used can be silicon nitride. In other embodiments, the dielectric is PZT or BZT, or other material with high relative permittivity, which notably allows the ratio Con/Coff to be increased and hence the transmission and isolation properties of the micro-switch to be improved, together with its characteristic switching times between the two states on and off.
RF micro-switches are being increasingly employed in order to improve the functionalities of radiofrequency circuits used notably in telecommunications systems. The aim is to obtain improved performance in terms of losses, noise, linearity and power consumption. They are also used in order to obtain high levels of compactness of components, and to reduce the costs of fabrication as much as possible.
There exist two types of micro-switches providing a switching function for radiofrequency signals on a transmission line: micro-switches in series with the radiofrequency transmission line and micro-switches in parallel with the radiofrequency transmission line.
When the micro-switches are of the series type, the application of a activation voltage under the membrane makes it go from an idle off state, open, to the on state, closed. The configuration of a micro-switch in series with a radiofrequency line is the following: the line is interrupted in the switching region, above which is the membrane. The membrane is isolated from electrical ground. The membrane does not have to withstand the radiofrequency power over its whole surface, its role not being to short-circuit the signal to ground but simply to connect two lines together by a capacitive effect.
When the micro-switches are of the parallel type, the application of a voltage to the control electrode makes it go from an idle on state, closed, to an open off state. The configuration of a micro-switch in parallel with a radiofrequency line is the following: the line RF is not interrupted in the switching region, above which is the membrane. The membrane is connected to electrical ground and must be capable of handling the power of the radiofrequency signal.
The operation is as follows: in the idle position, the micro-switch is in the on state, closed, which corresponds to a very low capacitance Coff, which does not affect the radiofrequency signal transmission. When it is in the low state, under the effect of an activation voltage under the membrane, the capacitance increases by a large factor, 100 for example. The capacitor then presents an impedance between the line and ground that is sufficiently low to shunt the radiofrequency line to electrical ground: the radiofrequency signal then flows from the line carrying the signal RF toward electrical ground via the membrane. The two portions of line, before and after the membrane, are then isolated: the micro-switch is in the off state.
The main advantages of these types of series or parallel micro-switches are essentially:                The fabrication techniques, which are derived from the conventional technologies for the fabrication of electronic integrated circuits. They allow the fabrication and integration to be simplified and, consequently, low fabrication costs to be obtained compared with those of other technologies, while at the same time guaranteeing a high level of reliability;        The very low electrical powers consumed, a few nanojoules being required for the activation;        The size. A micro-switch is thus fabricated within a surface area of the order of a tenth of a millimeter squared, allowing a high level of integration to be attained;        The performance in microwave applications. This type of micro-switch exhibits very low insertion losses, of the order of a tenth of a deciBel, well below those of devices providing the same functions.        
The desire for higher switching speeds, for higher RF power capabilities equal to or higher than the ten watt level, for wideband operation of at least 18 GigaHertz, for the smallest possible compactness and still at lower cost, for even longer lifetimes (number of switching operations), of the order of at least 1011 in order to meet the needs and evolutionary development of the market, notably of consumer markets such as for example mobile telephony, is driving the search for optimized structures and fabrication processes, the known micro-switch structures not completely meeting these needs.